The use of parts formed of composite material is currently very popular in the aerospace industry where the lightweight, high-strength characteristics of such parts are especially desirable. Composite parts are typically formed of high-strength anisotropic fibers such as graphite, embedded within a thermosetting resin matrix. A composite part, such as a wing panel, is fabricated by creating a stack of composite material layers on the surface of a mold that is contoured to create the desired panel shape. The mold may be made of metal or previously cured composite material. After the stack is laid up, the mold and stack of composite layers are placed in an autoclave where the resin is cured to create a homogeneous composite panel.
Composite panels tend to warp during curing. Although a minor amount of warping is tolerable, severe warping will result in rejection of the panel. The extent of warpage is typically assessed by measuring the differences between the shape of the mold and the shape of the finished panel.
One method of quantifying these shape differences is to measure the thickness of the gaps between the panel and the mold while the cured panel is positioned on the mold in the same location it was when it was cured. In the past, feeler gauges were used to measure these gaps. Feeler gauges are undesirable because they can only be used to measure gaps along the periphery of the panel. Warpage in the central area of the panel cannot be determined using feeler gauges.
Another prior technique used to measure the thickness of gaps between a composite panel and a metal mold utilizes a conventional eddy current sensor and ultrasonic thickness gauge. The eddy current sensor is placed on the surface of the composite panel that faces away from the mold. The sensor detects the location of the metal mold, thereby providing a measure of the distance between the sensor supporting surface of the panel and the mold. Next, the ultrasonic thickness gauge is positioned at the same location on the panel. The ultrasonic gauge provides a measurement of the thickness of the composite panel. The difference between the distance measured by the eddy current sensor and the thickness of the composite panel is a measure of the gap (hence warpage) at the point of measurement. This technique has the disadvantage that it requires the use of relatively sophisticated instruments that must be carefully positioned on the panel by the operator. When large composite panels are used, it is impossible for the operator to reach the central area of the panel by hand. Climbing onto the panel is unacceptable since the added operator's weight unacceptably distorts the panel shape. Thus, measurements in the center of large panels using this technique require the use of a bridge-like structure or some other apparatus to support the operator as the eddy current and ultrasonic measurements are being made. This technique also has the disadvantage that eddy current sensors are unusable when the mold is formed of a nonconductive (i.e., a composite) material.